28 Comparative Animal Physiology 



trations would be of interest, as well as investigation of the mechanism of 

 hypotonicity at high concentrations. 



During the molting cycle changes occur in osmotic concentration of the 

 blood of Pachygrapsus crassipes. In sea water of Ao= 1-975 the hard inter- 

 molt crabs showed a Ai of 1.327; in those about to molt Ai=2.601, in those 

 newly molted Ai=2.193, and in crabs at the paper-shell stage Ai=1.76. -- 

 Water and sugar content also changed during the molt. -^ 



In some parts of the world certain crabs live on lajid most of the time, 

 usually returning to salt water to breed. Their most interesting adaptations are 

 respiratory. Pearse^"^- ^'•'^ found the blood of several land crabs to be more 

 dilute than that of marine crabs while in one the concentration was about the 

 same. Their chitinous shells are good protection against evaporation. Hypo- 

 osmotic forms such as Uca can live longer in air than can poikilosmotic crabs 

 like Cancer, whose blood concentration increases markedly when they are 

 exposed to air. ^^- All crabs which show hypo-osmotic regulation are grapsoid 

 crabs, and all land crabs are also grapsoids. It is probable that the hypo-osmotic 

 condition permits regulation against increased osmotic concentration of water 

 in the branchial chamber when the crab is in air. ^-- However, many grap- 

 soid crabs remain in sea water, and it is unlikely that for these the ability to 

 maintain hypotonicity can have adaptive value. 



Good Regulation Permitting Life in Either Fresh or Sea Water. A few 

 crabs live successfully in either fresh or sea water. Young Eriocheir sinensis, 

 according to Krogh, ^^-^ ascend some of the rivers of Europe in the spring; 

 they mature in fresh water, but in the autumn the adults return to the sea to 

 breed. 



When Eriocheir is transferred to media of different tonicities there are no 

 volume changes but the blood concentration does rise and fall slightly. Ani- 

 mals Hving in North Sea water with Ao of 1.72 showed a Ai of 1.66, while 

 those in fresh water showed a Ai of 1.22. -^^ Eriocheir is hypertonic in dilute 

 and hypotonic in concentrated media (Fig. 14). However, Eriocheir differs 

 from Hemigrapsus, Pachygrapsus, and Uca in not having a steep and lethal 

 fall in its osmotic concentration in very dilute brackish or fresh water. 



The permeability of the gills of Eriocheir is extremely low. This crab takes 

 up iodine even more slowly than does the fresh-water crayfish (Fig 15). ^^'^ 



The urine output is at least 3.5 ml. /day from a 60 gm. Eriocheir in fresh 

 water. —'^ The urine is hypertonic or isotonic to the blood, -~^ irrespective 

 of the medium : 



Blood Urine Medium 



Ai Au 



1.18 1.23 freshwater 



1.85 1.91 Sea water of A" 2.23 



The salt composition of the urine differs from that of the blood. The kidneys, 

 therefore, put out much salt in the urine. 



In fresh water, Eriocheir replaces the salt it loses by actively absorbing some 

 salt through the gills. Krogh^^- observed that a crab loses salt rapidly when 

 transferred from fresh to distilled water. After 24 hours the salt loss in dis- 

 tilled water was at the rate of 108 mM/hr. with the kidneys open and 82 

 mM/hr. with the kidneys closed. The rate of ammonia loss was the same 

 whether the kidneys were open or closed; hence the loss of most of the chloride 



